Thermal recovery of petroleum hydrocarbons



Oct. 31, 1967 J L. HUH-T 3,349,843

THERMAL RECOVERY OF PETROLEUM HYDROCARBONS Filed March 26 1965 Fig. 1

#WE/V70@ JiMMIE L. HUITT United States Patent Oh ice 3,349,843 Patented Oct. 31, 1967 3,349,843 THERMAL RECOVERY F PETROLEUM HYDROCARBONS Jimmie L. Huitt, Glenshaw, Pa., assignor to Gulrl Research & Development Company, Pittsburgh, Pa., a corporation of Delaware Filed Mar. 26, 1965, Ser. No. 442,862

8 Claims. (Cl. 166-2) ABSTRACT 0F THE DISCLOSURE A process for recovering oil from a subterranean formation by forming a radial Ifracture. from a well bore through an oil-bearing formation, propping the fracture, injecting into the fracture a liquid containing fluid-loss additive to form a barrier against vertical movement of fluids, injecting gas into the oil-bearing formation below the barrier to heat the oil-bearing :for-mation, and producing oil from the formation below the fracture barrier.

This invention relates to an improved process for the thermal recovery of petroleum hydrocarbons `from underground oil bearing formations containing a substantially depleted gas cap or similar overlying high permeability structure.

Oil frequently occurs in underground oil formations in the presence of hydrocarbon gases under pressure. The primary recovery of this oil, in which pressurized gas is utilized to force the oil through the formation particles to the producing well, may be carried out for such time as surcient gas pres-sure is retained within the formation. When this natural pressure has substantially diminished, a major portion of the oil still remains in the reservoir. A suitable secondary recovery technique must be used to avoid the abandonment of this remaining oil. Furthermore, it is not uncommon to nd oil reservoirs which initially have insufficient pressure for economic primary recovery of the oil, and in this instance substantially all recovery is by a secondary recovery method.

Conducting in situ combustion between an injection well and a production well is a well-known method for the secondary recovery of reservoir oil. An advantage of this method is that it is useful for the recovery of thick viscous oils and oils with a high paraiiin content which occur in relatively low temperature formations. Unfortunately, there is a tendency for the injected oxidizing gas to rise above the oil zone to a low pressure gas cap or other overlying zone of higher gas permeability and pass directly to the production well or be dissipated within the reservoir without contacting the main bulk of the reservoir oil. When this occurs, in situ combustion is disrupted.

Sometimes viscous oils are produced by thermal stimulation `at the production well in a cyclic process. A heated fluid such as steam is first injected to heat the formation and reduce the viscosity of the oil. Steam injection is then terminated and the heated oil is produced. This cycle of thermal stimulation and production is repeated when the production rate decreases to a predetermined minimum as a result of the dissipation of the injected heat. Thermal stimulation is accomplished in another approach by injecting an oxidizing gas such as air into the production zone. In situ combustion is initiated and a portion of the reservoir oil is burned to heat the viscous oil in the formation and stimulate its production. The in situ combustion heats the oil 'and thereby makes it more flowable while the pressure resulting from the buildup of combustion gases and vaporized hydrocarbons Within the formation drives this heated oil to the production well during the production cycle. Gases such as these when injected into an oil formation tend to move upwardly in the formation. This tendency is intensilied in the vicinity of the well bore as a result of increased gas permeability resulting from displacement of the oil by the injected gas. Unfortunately, when using a thermal stimulation approach, the upward movement of injected gases as well as combustion gases may result in their escape to a gas cap or other loverlying permeable structure, if present. This problem is further heightened because the desired horizontal penetration of the injected gases may be from 5 to 100 times the distance from the point of injection to the gas cap. In short, I have determined that the occurrence `of an overlying, high gas permeability layer may reduce or destroy the effectiveness of a thermal recovery program.

In accordance with my invention,y I have discovered that thermal recovery processes can be effectively Iand etliciently carried out in the presence of an overlying gas cap or similar high permeability structure. This is accomplished by installing a barrier between the gas cap and the underlying oil. A fracture extending a substantial distance from the well bore is produced and propped and a barrier material is injected into the propped fracture. Upon installation of this fracture-barrier, a gas injected underneath into the oil bearing zone cannot short circuit to the gas cap. However, in positioning the fracture-barrier, a problem is created in 'attempting to avoid the inclusion of any portion of the gas-sapping permeable zone below the barrier and vavoid the trapping of oil above the barrier. Since it is not possible to precisely position a :fracture in a uniform and predetermined plane of great extent, and notwithstanding this in View 0f the fact that the gas-oil contact is a transition zone of decreasing oil saturation rather than a sharply defined interface, a Ifracture-barrier cannot be used for the complete separation of the oil and the more permeable Zone. I escape this dilemma by locating the fracture in the oil zone a finite distance below the gas-oil contact. A significant aspect of my invention is that I `am able to recover the oil thereby trapped above the barrier by employing a temporary barrier and utilizing the thermal stimulation effected by the heat front moving upwardly from the directly heated zone in the main body of the reservoir. In utilizing trny process the barrier is removed `following the thermal stimulation of the zone below the barrier and the heated oil trapped above the barrier is produced through the fracture. Therein, I provide a novel process for the thermal recovery of substantially all the recoverable reservoir oil without loss of either injected or combustion gases to an overlying permeable zone.

In producing the barrier a fracture is formed in a conventional manner and propped with a suitable propping agent. In the course of the fracturing operation a lowuid-loss agent is incorporated into the fracturing iiuid and is deposited on the two faces of the fracture. Subsequently the fracturing pressure is reduced and a pressure is maintained on the liquid in the fracture at a level sufiicient to prevent a backflow and to .maintain the low- Huid-loss agent on the faces of the fracture. The pressure maintained on the barrier liquid is greater than the existing formation pressure and the pressure of the gases subsequently injected for thermal stimulation but less than fracturing pressure. Because of the low-fiuid-loss conditions prevailing within the fracture, only a small quantity of injected liquid will flow into the formation beyond the outer periphery of the fracture. This fluid flow pro'- vides a pressure gradient which over the large areal extent of the fracture effectively shields the overlying gas cap from penetration by the injected gases. Y

This invention will be more speciiically disclosed by reference to the drawings in which FIGURE 1 is a vertical partial section through the borehole illustrating a tem- 3 porary barrier being utilized in conjunction with the thermal stimulation of a production well, and FIGURE 2 is a vertical section through an input and a production well illustrating a temporary barrier being utilized in conjunction with an ordinary forward drive in situ combustion process between the input and production well.

Referring first to FIGURE l a well penetrating overburden is completed in the oil bearing formation consisting of an oil zone 11 and a substantially depleted gas cap 12. The well casing 13 extending down to the underlying formation 14 is cemented in place in the usual manner. Impervious cap rock 15 defines the upper boundary of the productive formation. The viscosity of the oil is too high to permit an economical rate of flow to the production well at the temperature naturally occurring in the formation, and the gas pressure is too low to force the oil to the well bore at an economical rate. Fracture 16 extending a substantial radial distance into the formation is produced in a conventional manner using a suitable fracturing liquid. The fracture is located a sufficient distance below gas cap 12 to insure that no portion of this overlying gas bearing formation is included below the fracture and as a result a material portion 17 of the oil Zone as well as the gas-oil transition zone are isolated above the fracture. The fracture is propped with a suitable propping agent as shown and is pressured with a liquid containing a low-uid-loss agent to minimize the ow of liquid through the faces of the fracture. Tubing 18 extends through and below a packer 19 which provides a seal between tubing 18 and casing 13. Packer 19 is located below the fracture and separates the annulus 20 and the entrance tothe fracture from the tubing outlet and well perforations 21.

An oxidizing gas, preferably air, is pumped at a pressure greater than the reservoir pressure into the formation though perforations 2.1 and combustion is initiated in a conventional manner. Fingers of injected air project deeply into the formation sweeping out zone 22 with concomitant in situ combustion. The liquid in fracture-barrier 16 under pressure greater than the injected air confines the injected air, combustion gases and vaporiz/ed products against penetration of the gas cap and directs the injected air and combustion horizontally through the oil Zone as desired. At the stage of operation illustrated in FIGURE l it is apparent that the air would have already reached the gas cap in the absence of the fracture-barrier.

When significant heating of the formation has occurred, for example after several days or weeks, air injection is stopped and the air is purged from the tubing. The heated oil then flows through the perforations 21 with the aid of the pressure built up within the formation and is pumped to the surface. After a period of time oil flow reduces below an established minimum and the stimulation cycle is repeated. The heat front generated in zone 22 by the combustion will gradually move across the barrier and into the oil bearing zone 17 above the barrier. The fracture can be depressured during any production phase or when thermal stimulation has been nally terminated and the barrier liquid removed to produce the oil which is temporarily trapped above the barrier through the fracture to the well. Flow of oil from zone 17 to the fracture is enhanced by the natural pressure in the gas cap augmented by expansion from heating and by vaporized hydrocarbons.

In FIGURE 2 casing 25 of the injection well and casing 26 of the production well penetrate overburden 27 and are set through the oil bearing formation to underlying formation 28. The oil zone 29 is topped by a substantially depleted gas cap 30. Fracture 32 is formed in a conventional manner a signicant distance below the gas cap to insure that the `gas cap and transition zone is completely isolated above the fracture. As a result, a portion 33 of the oil is trapped above the fracture. The fracture extends radially a substantial distance from the injection well without penetrating the production well as indicated. The

i fracture is propped and pressured with a liquid containing a low-iluid-loss agent which deposits on the faces of the fracture preventing significant ow of liquid through the faces of the fracture into the formation.

Packer 37 seals off the outlet of air tubing 35 in cornmunication with the bottom of the injection well from annulus 36 in communication with fracture 32. Air is injected under pressure through tubing 35 and perforations 39 into the formation. Combustion is initiated in a conventional manner and a moving combustion front 40 is maintained to drive the oil bank ahead of it into the production well through perforations 41. This oil is delivered to the surface through production tubing 44. Fracture-barrier 32 prevents the air in permeable burned out zone 42 from entering the gas cap and short circuiting the combustion front and, as a result, maintains the combustion front below this barrier. Air injection and combustion is continued until thermal breakthrough occurs in production well 26. At this time air injection is stopped and the fracture depressured by withdrawing the barrier liquid from the fracture. The oil in zone 33, which has been made more mobile by the upward movement of a heat wave from the zone of combustion, is then produced for substantially complete recovery of the oil between the two wells.

If in situ combustion has been carried out for a period of time prior to the discovery that the injected air is short circuiting into a gas cap, it is possible to stop the process and install the barrier as described herein, and then reinitiate the combustion process in accordance with my invention. However, when it is known that a gas cap already overlies the oil zone, it is preferred that the barrier be installed prior to the initiation of in situ combustion since the channeling of injected air and breakthrough of the combustion front into the gas cap may alter the formation characteristics in such a manner as will interfere with the subsequent placement of the fracture.

Either water or a suitable liquid hydrocarbon may be used for the barrier liquid. When high temperatures are encountered under a particular set of operating conditions, it may be preferred to use a high boiling point hydrocarbon. It is recognized that a heat front will move upwardly away from the heated zone in the direction of the barrier. Since there is a small downward flow of barrier liquid, the upward movement of this heat front is delayed as it approaches the barrier by the downwardly flowing barrier fluid. This heat effect may also control the selection of the low-fluid-loss agent used to seal the fracture faces. Finely powdered inorganic materials such as ground silica, limestone, calcium sulfate, mica, sodium silicate, etc., which are insoluble in the barrier Huid may be utilized where resistance to heat is desired. When production from the fracture takes place after the heating operation, a suflicient portion of these materials will be washed away from the fracture by the outflow of reservoir oil to provide adequate permeability into the fracture. However, in situations where significant heating of the barrier liquid does not occur while the thermal operation is underway, low-fluid-loss agents which are heat sensitive may be utilized as well as the above-mentioned stable materials. When heat sensitive low-uid-loss materials are used under these conditions, the heat front moving up to the fracture after it is depressured, will destroy these barrier materials on the spot. Low-iluid-loss materials which will break down and lose their barrier properties in the presence of an elevated temperature include carbon black, carboxy methyl cellulose, asphalt, natural gums, and similar substances. These, however, are lmerely illustrative and many other useful examples are perfectly evident or well-known in the art.

In a specific example of my invention a casing is set through feet of oil bearing sand in the depth interval 3,220 to 3,300 feet. Approximately feet of gas sand overlie the oil sand. The oil cannot be produced at an economic rate since its viscosity is about 500 centipoises at the reservoir temperature of 138 F. The reservoir pressure is 900 p.s.i. The casing and cement are cut at a depth of 3,230 feet and the formation notched. A conventional fracturing liquid is injected into the notch at 3,650 p.s.i. and a fracture of extensive radius is produced. The fracture is propped with 6-8 mesh sand entrained in the fracturing liquid and 0.1 pound of silica flour per gallon of fracturing liquid is added to seal the faces of the fracture. The fracturing pressure is released and tubing equipped with a packer is run into the well to a depth of 3,265 feet with the unseated packer lying at 3,235 feet. The well is then perforated at the interval 3,265 to 3,280 feet. The tubing is next lowered until its bottom is at 3,285 feet and air is displaced down the annulus and up the tubing to clear the well of fracturing liquid to a depth of 3,285 feet. The tubing is then raised until its bottom is at 3,265 feet and the packer is seated at 3,235 feet to separate the fracture from the tubing outlet.

Gelled Water containing 0.1 pound of silica flour per gallon of water is pumped into the annulus at a rate which is suicient to maintain a pressure of 1,000 p.s.i. at the notch in the well bore. A burner is run into the tubing in the conventional manner and air is injected down the tubing to establish air permeability in the oil zone. This is accomplished when the air injection pressure increases to a level followed by a decrease to a substantially constant value of 1,100 p.s.i. Air injection is continued as the burner is ignited in a conventional manner to heat the air and raise the temperature in the oil zone to that point required to ignite the oil. Upon ignition of the oil the fuel supplyfto the burner is cut off and the pressure in the annulus is increased to maintain a pressure of 1,500 p.s.i. on the injection liquid at the notch while air injection is increased to 1,400 p.s.i. at the perforations. In this manner 25 million s.c.f. of air are injected into the oil bearing zone in a period of about two weeks.

When air injection lhas been completed, a volume of water in amount about three times the tubing volume is pumped down the tubing and into the perforations to clean the well of air. The burner is then removed from the tubing and the oil is produced through the perforations in the conventional manner. Concurrently the frac ture is depressured permitting the oil from the 3,220 to 3,230 zone, which has been heated by the heat front moving up through the barrier and into this zone, to be produced through the fracture. When the total production from the zone below the fracture `and above the fracture decreases to less than ten barrels per day, the cycle is repeated until thermal stimulation becomes economicah ly unattractive at which point substantially complete recovery of recoverable oil has been effected without interference of the gas cap.

The fracture-barrier will usually be placed in the upper portion of the oil bearing zone proximate to the gas-oil Vsitu combustion. However,

hydrocarbons, may be utilized in favorable circumstances to drive the heated oil in the zone above the fracture down into the fracture for production into the well.

This invention has been disclosed With specific description of well stimulation by in situ combustion. In addition it is useful for any technique of thermal stimulation involving injection of a fluid into the oil zone when the loss of the injected fluid into an overlying permeable zone must be prevented and it is not desired to abandon recoverable reservoir oil. This includes thermal stimulation by steam injection, injection of preheated gases such as hot products of combustion, or mixtures of steam and heated gases. Air is the preferred oxidizing gas for in other oxidizing gases are known in the art for this use including oxygen and oxygenenriched air. Therefore, in the claims, air is used generically to mean air itself as well as other useful combustionsupporting gases. Furthermore, the recovery process is generally useful whether it is applied lto push-pull stimulation of a single well as described, or to forward or reverse drive in situ combustion in multiple well operation.

It is to be understood that the above disclosure is by way of specific example and that numerous modifications and variations are available to those of ordinary skill in the art without departing from the true spirit and scope of my invention.

I claim:

1. A process for the recovery of oil from an underground oil bearing formation contacting an overlying zone of higher permeability than said oil bearing formation in which a gas in injected into the oil bearing formation for thermal stimulation of the oil which comprises,

contact. However, it may be placed anywhere in the i oil bearing formation as long as it is above the oil zone under thermal stimulation. That is, in a thick oil formation capped with a gas permeable zone, the barrier may be placed below the midpoint of the oil bearing portion of the formation to insure significant radial stimulation of the reservoir Without escape of injected gas to the overlying permeable zone. In this instance it may be desired to sequentially install -additional barriers at higher levels for the stepwise thermal recovery of oil by my recovery method.

Frequent use has been made throughout of the expressions substantially depleted gas cap and highly permeable overlying zone. This refers to the substantially oil-free upward extension of the oil bearing formation. The interstices of this zone under conditions prevailing in the reservoir are filled with gas rather than oil. This gas, whose pressure is increased by the heat front which moves up from the directly heated zone and amplified by vaporized forming a radial fracture in the oil bearing formation above the point of gas injection,

propping said fracture,

injecting a liquid containing a low-fluid-loss agent into said fracture at a pressure greater than the formation pressure and less than fracturing pressure to form a barrier against vertical movement of fluids across said fracture,

injecting said gas into the oil bearing formation below said barrier to heat said oil bearing formation and thermally stimulate said oil,

producing oil from the oil bearing formation lying below said fracture, depressuring said fracture from saidfracture, and producing oil from the oil bearing formation lying above said fracture through said fracture.

2. A process in accordance with claim 1 in which said gas is air.

3. A process in accordance with gas is selected from steam, thereof.

4. A process in accordance with claim 1 in which said liquid and low-fluid-loss agent are stable at elevated temperatures.

5. An in situ combustion process for the recovery of and withdrawing said liquid claim 1 in which said hot inert gases and mixtures oil from an underground oil bearing formation penetrated by a well and contacting an overlying zone of higher permeability than said oil bearing formation in which air is injected into the oil bearing formation to support in situ combustion of the oil which comprises,

forming a radial fracture in the oil bearing zone above the point of air injection, propping said fracture and injecting a liquid containing a low-fluid-loss agent into said fracture at a pressure greater than the formation pressure and less than fracturing pressure to form a barrier against vertical movement of fluids across said fracture, injecting air into the oil formation below said barrier to support in situ combustion to heat said oil bearing formation, producing oil from the oil bearing formation lying below said fracture,

depressuring said fracture and withdrawing said liquid from said fracture, and

.producing oil from the oil bearing formation lying above said fracture through said fracture.

6. A process in accordance with claim 5 in which said liquid and said low-uid-loss agent are stable in the presence of elevated temperatures.

7. An in situ combustion process for the recovery of oil from an underground oil bearing formation penetrated by at least one input well and one production well 1 and contacting an overlying Zone of higher permeability than said oil bearing formation in which air is injected into the oil bearing formation to support in situ combustion of the oil which comprises,

forming a radial fracture in the oil bearing zone at the input well above the point of air injection and stopping short of the production well, propping said fracture and injecting a liquid containing a low-uid-loss agent into said fracture at a pressure greater than the formation pressure and less than fracturing pressure to form a barrier against vertical movement of fluids across said fracture, injecting air into the oil formation at the input well 8. A process in accordance with claim 7 in which said liquid and said low-uid-loss agent are stable in the presence of elevated temperatures.

References Cited UNITED STATES PATENTS Parker 166-42 X Huitt 166--11 X Huitt 166-29 Warren 166--29 X Papaila 166-42 CHARLES E. OCONNELL, Primary Examiner.

NILES` C. BYERS, Examiner. 

7. AN IN SITU COMBUSTION PROCESS FOR THE RECOVERY OF OIL FROM AN UNDERGROUND OIL BEARING FORMATION PENETRATED BY AT LEAST ONE INPUT WELL AND ONE PRODUCTION WELL AND CONTACTING AN OVERLYING ZONE OF HIGHER PERMEABILITY THAN SAID OIL BEARING FORMATION IN WHICH AIR IS INJECTED INTO THE OIL BEARING FORMATION TO SUPPORT IN SITU COMBUSTION OF THE OIL COMPRISES, FORMING A RADIAL FRACTURE IN THE OIL BEARING ZONE AT THE INPUT WELL ABOVE THE POINT OF AIR INJECTION AND STOPPING SHORT OF THE PRODUCTION WELL, PROPPING SAID FRACTURE AND INJECTING A LIQUID CONTAINING A LOW-FLUID-LOSS AGENT INTO SAID FRACTURE AT A PRESSURE GREATER THAN THE FORMATION PRESSURE AND LESS THAN FRACTURING PRESSURE TO FORM A BARRIER AGAINST VERTICAL MOVEMENT OF FLUIDS ACROSS SAID FRACTURE, INJECTING AIR INTO THE OIL FORMATION AT THE INPUT WELL BELOW SAID BARRIER TO CAUSE A FLAME FRONT TO MOVE IN THE DIRECTION OF SAID PRODUCTION WELL, PRODUCING OIL FROM THE OIL BEARING FORMATION LYING BELOW SAID FRACTURE AT SAID PRODUCTION WELL, DEPRESSURING SAID FRACTURE AND WITHDRAWING SAID LIQUID FROM SAID FRACTURE, AND PRODUCING OIL FROM THE OIL BEARING FORMATION LYING ABOVE SAID FRACTURE THROUGH SAID FRACTURE INTO SAID INPUT WELL. 